To increase fabric knitting versatility the technique of equipping a circular knitting machine with yarn changing apparatus to add yarns of other colors or types in the knitting fabric is widely adopted in textile industry. Reference of such technique can be found in U.S. Pat. Nos. 7,036,343, 7,055,348, 6,655,176 and 5,218,845.
A yarn changing apparatus 10 is disclosed in U.S. Pat. No. 5,218,845 as shown in FIGS. 1A through 1D. It mainly comprises a yarn feeding plate 11, a yarn clipping plate 12 and a spacer 13. In a yarn non-feeding condition, the yarn feeding plate 11 and yarn clipping plate 12 are retracted in the yarn changing apparatus 10, and the yarn clipping plate 12 has a hook 121 at the front end to clip a yarn 15 between the yarn clipping plate 12 and spacer 13. In a yarn feeding condition, the yarn feeding plate 11 is driven forwards with a front end 111 moved out (referring to FIG. 1B) to pull the yarn 15 outwards to a yarn feeding position so that a knitting needle 14 can hook the yarn to do knitting operation. When the yarn feeding plate 11 is move outwards its tail end 112 hits a first strut 122 of the yarn clipping plate 12 to move the yarn clipping plate 12 in the same direction of the yarn feeding plate 11 until the yarn feeding plate 11 is moved to the yarn feeding position, then the hook 121 of the yarn clipping plate 12 releases the yarn 15 (referring to FIG. 1C).
When change to other yarn is desired, the yarn feeding plate 11 previously moved to the yarn feeding position is retracted into the yarn changing apparatus 10, and a nose 113 of the yarn feeding plate 11 hits a second strut 123 of the yarn clipping plate 12 to move the yarn clipping plate 12 back into the yarn changing apparatus 10 (referring to FIG. 1D). When the retraction is finished, the yarn 15 is severed by scissors and clipped between the hook 121 and spacer 13 again (return to the condition shown in FIG. 1A).
The aforesaid yarn changing apparatus can provide multiple yarn colors or types. Take a commonly adopted one with four colored yarns as an example, each yarn changing apparatus includes four sets of yarn feeding plates and yarn clipping plates arranged in a juxtaposed manner to change four types of yarns.
To increase the number of changeable yarns, more yarn feeding plates and yarn clipping plates are added to the yarn changing apparatus, such as six sets of yarn feeding plates and yarn clipping plates for a six-colored-yarn changing apparatus. However, with increasing number of changeable yarns, more yarn feeding plates and yarn clipping plates have to be added, and the volume of the yarn changing apparatus increases. Therefore, the number of the yarn changing apparatus installable on one circular knitting machine would be reduced. On the other hand, to maintain the original installed number, the size of the yarn changing apparatus has to be maintained, and the composed elements have either to be shrunk or made thinner. This results in a lower mechanical strength and decreased reliability. Moreover, when moving forwards or backwards, the yarn feeding plate needs an additional stroke to drive the yarn clipping plate. With more yarn changing movements taking place on one yarn changing apparatus, knitting speed would be decreased.
In order to maintain the design of the general yarn changing apparatus and also achieve more yarn changing effect to increase knitting diversity of the circular knitting machine, U.S. Pat. No. 7,073,355 discloses a knitting machine with at least one striping attachment which mainly includes a bracket to hold knitting tools, a cam mechanism and a yarn striping means. The cam mechanism has at least one loop forming zone in which the selected knitting tools are moved to a yarn drawing position. The yarn striping means feeds the selected yarn to a knitting tool already at the yarn drawing position. The yarn striping means includes at least a first yarn changing apparatus and a second yarn changing apparatus that are selectively fed with a yarn through an engagement means, then the knitting tool already at the yarn drawing position is deployed in the loop forming zone.
Although U.S. Pat. No. 7,073,355 provides the engagement means to selectively bring the yarn of the first or second yarn changing apparatus into the loop forming zone, the yarn changing apparatus and knitting needles are spaced at a narrow space, and the engagement means has to be installed and operate in that narrow space, installation is difficult. In the yarn non-feeding condition the engagement means becomes a spatial obstacle for knitting. Moreover, the yarn engagement means holds the yarn through a clipping means. Tight or loose condition of the clipping means affects whether the yarn can be carried to the loop forming zone to be knitted by the knitting tool. Too strong of clipping force makes the yarn difficult to leave the engagement means, while inadequate clipping force makes the yarn leaving the engagement means too early. All this hinders yarn changing and affects the quality of knitted fabrics.